Improved selective catalytic reduction system and method

ABSTRACT

A selective catalytic reduction (SCR) system for treating exhaust gases in an exhaust passage is provided. The system comprises a first catalyst for converting urea to ammonia, the first catalyst being located in the passage and having an upstream face and a downstream face. The upstream face is at a right angle to an axial flow path (A) through the passage. The system further comprises a second catalyst for converting NOx to nitrogen gas and water in the presence of ammonia, with the second catalyst being located in the passage downstream of the first catalyst. A diesel exhaust fluid dosing unit is located upstream of the first catalyst. The dosing unit comprises a nozzle arranged so as to inject DEF directly onto the upstream face of the first catalyst.

FIELD OF THE INVENTION

The present invention relates to the field of internal combustionengines, and more specifically is an improved selective catalyticreduction (SCR) system and method for treating nitrogen oxides in dieselengine exhaust gases.

BACKGROUND OF THE INVENTION

SCR systems typically employ a SCR catalyst which converts nitrogenoxides (NOx) in the exhaust gas into nitrogen gas and water in thepresence of ammonia. The ammonia is typically obtained by injectingaqueous urea (commonly known as diesel exhaust fluid, or DEF) into theexhaust stream, whereupon the urea undergoes a hydrolysis and/orthermolysis within the exhaust passage whereby ammonia is produced. Theammonia passes into the SCR catalyst where it reacts with the exhaustgas, wherein any nitrogen oxides (NOx) present in the exhaust gas areconverted to nitrogen and water before passing out of the exhaust intothe atmosphere.

To assist with, or speed up, the conversion of the DEF into ammonia itis known to employ an additional catalyst upstream of the SCR catalystin the exhaust passage.

This additional catalyst may be a hydrolysis catalyst, with the DEFinjected into the exhaust stream flowing into the catalyst andundergoing a catalytic reaction whereby the DEF is converted to ammoniawhich then flows downstream into the SCR catalyst to effect theaforementioned conversion of the NOx. Alternatively, an impaction mixermay be employed upstream of the SCR catalyst, where the impaction mixerprovides a surface for the evaporation, thermolysis and hydrolysisreactions to take place.

One disadvantage of using an impaction mixer upstream is that localisedcooling of the mixer can occur at higher DEF dosing rates and/or lowerexhaust temperatures and flow rates. Added to the limited surface areaof the mixer this can lead to the accumulation of liquid on the mixerand subsequent inhibition of the urea to ammonia conversion reactions.Furthermore, this inhibition increases the likelihood of intermediateby-product reactions taking place, resulting in the formation of ureadeposits both on the mixer and in the downstream exhaust pipework.

One solution to this problem has been proposed in US2009/0324453, whichdiscloses a SCR system in which an upstream pyrolysis and/or hydrolysiscatalyst has an upstream face which is at an angle of 20-70 degrees tothe direct of axial flow. The purpose of this arrangement is to reducethe distance between a DEF injector and the upstream face of thecatalyst and use catalysts to improve conversion of urea to ammonia. Onedisadvantage of such an arrangement is that the catalyst has to becreated specifically for this purpose with the angled face, or else aconventional catalyst has to be cut to create the angled face. Eitherway, this increases the cost of manufacture. In addition, the angledface of the catalyst may extend the overall length of at least 30-50% ofthe cells within the catalyst, which can lead to an undesirable increasein back pressure within the exhaust system.

It is an aim of the present invention to obviate or mitigate one or moreof the aforementioned disadvantages.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided aselective catalytic reduction (SCR) system for treating exhaust gases inan exhaust passage, the system comprising:

-   -   a first catalyst for converting urea to ammonia, the first        catalyst being located in the passage and having an upstream        face and a downstream face, wherein the upstream face is at a        right angle to an axial flow path through the passage;    -   a second catalyst for converting NOx to nitrogen gas and water        in the presence of ammonia, the second catalyst being located in        the passage downstream of the first catalyst; and    -   a diesel exhaust fluid (DEF) dosing unit located upstream of the        first catalyst, the dosing unit comprising a nozzle arranged so        as to inject DEF directly onto the upstream face of the first        catalyst.

According to a second aspect of the invention there is provided a methodof treating exhaust gases in an exhaust passage, the method comprisingthe steps of:

-   -   locating a first catalyst for converting urea to ammonia in the        passage, the first catalyst having an upstream face and a        downstream face, wherein the catalyst is located such that the        upstream face is at a right angle to an axial flow path through        the passage;    -   locating a second catalyst in the passage for converting NOx to        nitrogen gas and water in the presence of ammonia, the second        catalyst being located in the passage downstream of the first        catalyst;    -   locating a diesel exhaust fluid (DEF) dosing unit upstream of        the first catalyst, the dosing unit comprising a nozzle; and    -   injecting DEF from the nozzle directly onto the upstream face of        the first catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the following drawings:

FIG. 1 is a sectional view of a first embodiment of a selectivecatalytic reduction system; and

FIG. 2 is a sectional view of a second embodiment of a selectivecatalytic reduction system.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of the core components of a first embodimentof a selective catalytic reduction (SCR) system, which is located in anexhaust pipe or passage 10 of a vehicle. Exhaust gas flows in thedirection shown by arrow E from a diesel engine of the vehicle towardsan exhaust tailpipe (neither shown) and from there to atmosphere. Theexhaust gas may first flow through a known diesel oxidation catalystbefore reaching the components of the SCR system shown in FIG. 1, and itmay also flow through a known ammonia slip catalyst between the systemand the tailpipe. However, neither of these components is shown here arethey are not core components of the present SCR system.

The SCR system, generally designated 20, comprises a first catalyst 30for converting urea to ammonia. The first catalyst 30 comprises apermeable substrate, which preferably has a plurality of cells 32contained therein and extending along the length of the substrate. Thesubstrate preferably has cell density of at least 50 cells per squareinch. The permeable substrate may be a metallic substrate such as, forexample, stainless steel or alternatively it may be a ceramic substratesuch as, for example, cordierite. The catalyst substrate may beuncoated, whereby the substrate is effectively acting as an impactiondevice facilitating thermolysis and/or pyrolysis reactions within theurea. Alternatively, the substrate may be coated with a hydrolysiscatalyst coating or washcoat of a known type so as to promote hydrolysisreactions within the urea.

The first catalyst 30 is located in the passage 10 and has an upstreamface 31 and a downstream face 33. The upstream face 31 is substantiallyat right angles to an axial flow path A through the passage 10. In otherwords, at the location of the first catalyst 30 in the passage 10 theupstream face 31 is substantially perpendicular to an internal wall 12of the passage.

The system 20 further comprises a second catalyst 40 for convertingnitrogen oxides (NOx) to nitrogen gas and water in the presence ofammonia. The second catalyst 40 may be a SCR catalyst of a known type,and may include a particulate filter (not shown) immediately upstreamthereof. The second catalyst 40 is located in the passage 10 downstreamof the first catalyst 30.

Located upstream of the first catalyst 30 is a diesel exhaust fluid(DEF) dosing unit 50. The dosing unit 50 comprises a controller or ECU52 for controlling the dosing rate of the unit, and is connected to areservoir 54 containing a supply of DEF. The dosing unit also comprisesa nozzle 56 connected to the reservoir 54, where the nozzle is arrangedso as to inject DEF directly onto the upstream face 31 of the firstcatalyst 30. The term “directly” in this context means that a spray ofDEF droplets 58 are injected from the nozzle 56 into the passage 10 insuch a way as to impact or impinge upon the upstream face 31 of thefirst catalyst, as opposed to the droplets evaporating prior to reachingthe first catalyst.

The system 20 may further comprise an exhaust mixer 60, which includes aplurality of flow-directing vanes 62 for generating turbulence andensuring the thorough mixing of the exhaust gases and ammonia as theflow travels downstream into the second catalyst 40. The exhaust mixer60 is located in the passage 10 downstream of the first catalyst 30, andis preferably close-coupled to the downstream face 33 of the firstcatalyst. By “close-coupled” it is meant that the mixer 60 is attachedto the downstream face 33 or at least is located in close proximity(i.e. less than 100 mm) to the downstream face.

FIG. 2 shows a sectional view of the core components of a secondembodiment of a selective catalytic reduction (SCR) system, generallydesignated 20′, which is located in an exhaust pipe or passage 10 of avehicle. The second embodiment of the system shares the majority of thecomponents of the first embodiment. These shared components also sharethe same reference numbers as used above and will not be described againhere.

The difference between the first and second embodiments of the systemrelates to the positioning of the nozzle 56′ of the DEF dosing unit 50.In this second embodiment the nozzle 56′ is again arranged so as toinject DEF directly onto the upstream face 31 of the first catalyst 30.However, in this embodiment the nozzle 56′ does this from a position inwhich it is substantially co-axial with the axial flow path A. In otherwords, the nozzle 56′ has a central axis N which is substantiallyco-axial with the axial flow path A. Thus, the spray 58′ of DEF dropletsin the second embodiment leaves the nozzle 56′ from a position directlyin front of the upstream face 31 of the first catalyst, as opposed tofrom an angled position to the side of the first catalyst. The distancebetween a tip of the nozzle 56′ and the upstream face 31 is proportionalto a spray cone angle of the DEF leaving the nozzle tip. The tip of thenozzle is preferably 125-175 mm from the upstream face, with a spraycone angle of 30-60°.

INDUSTRIAL APPLICABILITY

A method of treating exhaust gases in an exhaust passage will now bedescribed, with reference to the components of the SCR systems shown inFIGS. 1 and 2. The method comprises locating the first catalyst 30 forconverting urea to ammonia in the passage 10, and locating the secondcatalyst 40 in the passage downstream of the first catalyst 30 forconverting NOx to nitrogen gas and water in the presence of ammonia. Themethod may also include an initial step of coating the permeablesubstrate of the first catalyst with a hydrolysis coating or washcoat.The DEF dosing unit 50 is located upstream of the first catalyst, andmay be located as shown in FIG. 2 whereby the DEF nozzle issubstantially co-axial with the axial flow path A. Preferably, thenozzle 56, 56′ is arranged such that the DEF spray 58,58′ impinges upona surface area S1 of the upstream face 31 of the first catalyst 30. Thesurface area S1 covered by the DEF spray 58,58′ is preferably at least50% of the total surface area S of the upstream face 31.

The DEF spray may 58,58′ preferably comprises droplets having a Sautermean diameter of less than 50 microns. The spray 58,58′ is preferablyformed as a cone, where the peak spray mass-flux impacting any point onthe upstream face 31 does not exceed 2.5 times the average mass-fluximpacting the upstream face 31, when measured over an area that includes95% of the total impacting spray mass-flux.

As exhaust gas flows in direction E, the DEF dosing unit 50 will injectDEF from the nozzle 56,56′ such that DEF droplets 58,58′ are sprayeddirectly onto the upstream face 31 of the first catalyst 30. If thepermeable substrate forming the first catalyst is not coated with ahydrolysis coating then the urea landing on the substrate will undergo athermolysis or pyrolysis reaction such that the urea is converted intoammonia. If the substrate is coated with a hydrolysis coating then theurea will undergo a hydrolysis reaction whereby it is converted intoammonia.

The ammonia and exhaust gas exiting the downstream face 33 of the firstcatalyst then enter the exhaust mixer 60, where the vanes 62 of themixer swirl and/or guide the flow so as to ensure a thorough mixing ofthe ammonia and exhaust gas before the flow enters the second catalyst40. In the second catalyst any NOx present in the exhaust gas isconverted to nitrogen gas and water in the presence of the ammonia.These harmless nitrogen and water components are then carried out of theexhaust tailpipe to the atmosphere.

By spraying the DEF directly onto the upstream face of the firstcatalyst the urea to ammonia conversion rate of the system is improvedin comparison to systems in which the DEF is simply injected into theexhaust gas stream. It also reduces the chances of DEF deposits formingon internal surfaces of the exhaust passage upstream of the firstcatalyst. In the embodiments where the catalyst upstream face is actingas an impaction surface directly spraying the DEF onto that surfacecauses the DEF droplets to break up on impact, reducing the likelihoodof any build-up of liquid on the catalyst and the undesirable cooling ofthose surfaces which may occur as a consequence. The desired evaporationof the DEF is further enhanced when the optional hydrolysis catalystcoating is applied to the substrate of the first catalyst.

Furthermore, performing this direct injection on an upstream catalystface which is perpendicular to the passage and axial gas flow means thatno additional manufacturing steps are required to produce a catalystwhose upstream face is angled relative to the axial flow. Using acatalyst whose upstream face is perpendicular also means that there isno additional back pressure generated in the passage due to the longerlength of at least a portion of the cells in the catalyst.

Placing the optional flow mixer downstream of the first catalyst doesnot just ensure the mixing of the ammonia and exhaust gases.Additionally, the surfaces of the mixer also act as a secondaryevaporation location for any DEF spray droplets which exit thedownstream face of the first catalyst without having been evaporated.

Modifications and improvements may be incorporated without departingfrom the scope of the present invention as defined by the accompanyingclaims.

1. A selective catalytic reduction (SCR) system for treating exhaustgases in an exhaust passage, the system comprising: a first catalyst forconverting urea to ammonia, the first catalyst being located in thepassage and having an upstream face and a downstream face, wherein theupstream face is at a right angle to an axial flow path through thepassage; a second catalyst for converting NOx to nitrogen gas and waterin the presence of ammonia, the second catalyst being located in thepassage downstream of the first catalyst; and a diesel exhaust fluid(DEF) dosing unit located upstream of the first catalyst, the dosingunit comprising a nozzle arranged so as to inject DEF directly onto theupstream face of the first catalyst.
 2. The system of claim 1, whereinthe first catalyst comprises a permeable metallic substrate.
 3. Thesystem of claim 1, wherein the first catalyst comprises a permeableceramic substrate.
 4. The system of claim 2, wherein the substrate iscoated with a hydrolysis catalyst.
 5. The system of claim 1, wherein thenozzle is substantially co-axial with the axial flow path through thepassage.
 6. The system of claim 1, further comprising an exhaust gasmixer located in the passage downstream of the first catalyst.
 7. Thesystem of claim 6, wherein the exhaust gas mixer is close-coupled to thedownstream face of the first catalyst.
 8. A method of treating exhaustgases in an exhaust passage, the method comprising the steps of:locating a first catalyst for converting urea to ammonia in the passage,the first catalyst having an upstream face and a downstream face,wherein the catalyst is located such that the upstream face is at aright angle to an axial flow path through the passage; locating a secondcatalyst in the passage for converting NOx to nitrogen gas and water inthe presence of ammonia, the second catalyst being located in thepassage downstream of the first catalyst; locating a diesel exhaustfluid (DEF) dosing unit upstream of the first catalyst, the dosing unitcomprising a nozzle; and injecting DEF from the nozzle directly onto theupstream face of the first catalyst.
 9. The method of claim 8, whereinthe first catalyst comprises a permeable metallic substrate.
 10. Themethod of claim 8, wherein the first catalyst comprises a permeableceramic substrate.
 11. The method of claim 9, further comprising thestep of coating the substrate with a hydrolysis catalyst.
 12. The methodof claim 8, wherein the step of locating the dosing unit compriseslocating the nozzle such that it is co-axial with the axial flow paththrough the passage.
 13. The method of claim 8, wherein the upstreamface of the first catalyst has a first surface area, and the nozzleinjects the DEF such that a DEF spray impinges upon a second surfacearea of the upstream face which is at least 50% of the first surfacearea.
 14. The method of claim 8, further comprising the step of locatingan exhaust gas mixer in the passage downstream of the first catalyst.15. The method of claim 14, wherein the step of locating the exhaust gasmixer in the passage comprises close-coupling the exhaust gas mixer tothe downstream face of the first catalyst.